INTRODUCTION Although medical-toxicological literature contains what may be considered a plethora of information on clinical lead poisoning in man and on the effects of short term exposures of experimental animals to large amounts of lead@ little information is available on the toxic effect of prolonged exposures to small amounts of lead. This research was designed to evaluate the effects of ingestion of small amounts of lead by laboratory animals over a substantial part of their lifespan. Four mammalian species@ rats@ dogs@ monkeys and rabbits@ were studied. In addition to the common chronic toxicity tests@ the program included studies on reproduction@ teratology@ behavior@ carcinogenicity@ and metabolism as well as special enzyme and electron microscopy studies. Lead was administered perorally to the animals as lead acetate@ Pb(C2H3O2)2??3H2O. Dietary levels and dosages were calculated on the basis of actual lead@ this being 54.6% of weight of the acetate. The amount of lead administered to the animals was far in excess of the usual intake by man. For example@ on a per body weight basis@ the dogs and rats on the lowest dietary level ingested approximately 100 times the amount ingested by an average American adult. In the monkeys the intake was 300 times as high as that of the average American adult. Where applicable@ the life-table technique was used for the statistical analysis of survival and analysis of variance (F-Test) at the 5% probability level was used for the other criteria of toxic effect.